This invention relates to a vacuum shield device of an electron beam welding apparatus.
Electron beam welding is an electric fusion welding process in which an electric current in the form of a stream of beam of electrons is used to melt a workpiece by impingement on it. Generally, electron beam welding is carried out in vacuum. Thus it is necessary when electron beam welding is carried out to place the portion of the workpiece to be joined by welding in an evacuated space. To this end, it is usual practice to place the whole of the workpiece in the vacuum chamber when electron beam welding is carried out. However, it is also known to use an electron beam welding apparatus of the type in which a portion of the workpiece to be joined by welding and its vicinity alone are placed in an evacuated space sealed by a vacuum shield device without using a vacuum chamber of large size. The electron beam welding apparatus of the type described includes a vacuum shield device located in the lower portion of the apparatus and brought into contact with the workpiece to carry out electric beam welding in vacuum. Generally, the vacuum shield device is constructed to have a flat undersurface. When the vacuum shield device having a flat undersurface is used for effecting butt welding on components having flat surfaces, it is possible to maintain the weld in vacuum by merely placing the device on the workpiece. However, when the workpiece has a surface which is not flat but convex, for example, as in a tubular member, a gap will be formed between the flat undersurface of the vacuum shield device and the convex surface of the workpiece to be welded, thereby making it impossible to maintain the weld in vacuum. In order to eliminate this trouble, it has hitherto been usual practice to use a vacuum shield spacer conforming in the shape of its undersurface to the shape of the workpiece to be welded which is connected to the bottom of the vacuum shield device to keep the weld in vacuum when electron beam welding is carried out. A single vacuum shield spacer could not serve the purpose of supplementing the vacuum shield device in handling all the types of workpiece of multifarious shapes. Thus it is necessary to have in reserve a large number of vacuum shield spacers of different shapes or fabricate a suitable spacer each time when the need arises. This not only increases the cost of the electron beam welding apparatus but also makes an electron beam welding operation a time-consuming process.
In order to point out the disadvantages of the prior art, one example of the vacuum shield device will be outlined by referring to FIGS. 1 to 3. In FIG. 1, the electron beam welding apparatus comprises a welding machine 1, a vacuum shield device 2 located below the welding machine 1, a vacuum shield spacer 3 located below the vacuum shield device 2, and a backing member 5 arranged on the back of a workpiece 4. The vacuum shield device 2 has a substantially planar undersurface and when the workpiece 4 is not planar at its surface but convex, as shown in FIG. 1, a gap will be formed between the convex surface of the workpiece 4 and the flat undersurface of the vacuum shield device 2 if the latter is placed on the former, thereby making it impossible to maintain a weld 6 of the workpiece 4 in vacuum. Thus it is necessary to interpose between the vacuum shield device 2 and the workpiece 4 the vacuum shield spacer 3 which conforms in the shape of its undersurface to the surface of the workpiece 4. As shown in FIGS. 2 and 3, the vacuum shield spacer 3 has O-rings 7 or other rings, not shown, formed of silicon rubber or other heat-resisting material provided on the undersurface thereof either singly or in combination. When in position, the O-rings 7 are forced against the surface of the workpiece 4 and suitably deformed. By the vacuum shield spacer 3 cooperating with the backing member 5, the weld 6 can be kept in vacuum while seam welding is being performed on the workpiece 4.
In the vacuum shield spacer 3 of the aforesaid construction, the spacer 3 can accommodate a change in the surface of the workpiece 4 only to the extent of the deformation of the O-rings 7. The range of dimensions of the workpiece 4 in which the vacuum shield spacer 3 can have application is small, and in actual practice it is necessary to prepare and have in reserve a large number of vacuum shield spacers 3 or to fabricate a new vacuum shield spacer 3 each time there is a change in the dimensions of the workpiece 4.